1. Field of the Invention
This invention relates to a four wheel drive system for driving both front wheels and rear wheels of a vehicle by virtue of a power plant.
2. Description of Related Art
In four wheel drive vehicles, it has been known that a cut-off clutch may be provided between the power plant and either a front wheel driving system or a rear wheel driving system. The cut-off clutch is disengaged to establish a two wheel driving system when the four wheel drive condition is not necessary to be maintained so that a power loss is reduced to improve fuel consumption efficiency.
Japanese Patent Public Disclosure No. 62-181916, laid open to the public in 1987, proposes a four wheel drive system in which driving power is transmitted from the power plant to the rear wheels. The proposed four wheel drive system is provided with hydraulic clutches (hereinafter referred to as wheel clutches) on a driving mechanism for the right and left rear wheels, respectively, to control driving torque for the right and left rear wheels. As a result, torque distribution between the right and left rear wheels is controlled to improve controllability of a vehicle in a steering condition.
The wheel clutch can be provided in either the front or rear wheel drive mechanism.
Thus, appropriate torque distribution can be made between the front and rear drive system and/or between the right and left wheels to improve controllability.
It should be noted, however that a torque shock may occur when the cut-off clutch and the wheel clutches are engaged to transfer from the two wheel drive condition to the four wheel drive condition. This is because torque from the power plant is abruptly transmitted to the wheels.